In a known apparatus of this type (DE-OS No. 26 35 623), the carrying elements comprise carrying sections embedded in at least two outer ends of the corrugations, with a web fixed to the center of each carrying section and extending through an appropriate opening in the insulating mat to the outer side of the insulating block where it passes through an opening in a fixing bar arranged transversely to the fibers of the mat and, by being bent into the plane of the fixing bar is made capable of being joined to the latter. The fixing bar itself can be slid laterally into a C-shaped holding element which can be joined to the furnace wall, an opening being provided in the C-shaped holding element for a fixing bolt or similar means for the purpose of joining the C-shaped holding element to the furnace wall. An opening is likewise provided in the fixing bar which can be slid into the C-shaped holding element, said opening being capable of being lined up with the opening in the C-shaped holding element, thus making the fixing means that pass through the C-shaped holding element accessible when a corresponding opening has been made in the insulating mat at the opposite end to the above openings. With this particular configuration, a defect in any insulating block makes it necessary to shut down the furnace and allow it to cool in order then to be able to unscrew the fixing bolts passing through the insulating mat in the manner described above from the inside of the furnace, and having removed one insulating block it also being possible to remove and replace other insulating blocks by sliding the fixing bars sideways out of the C-shaped holding elements.
In another known apparatus (DE-OS No. 22 31 658), a plurality of fiber mats are joined by carrying rods that penetrate all plates and which are bent at right angles at their ends, the carrying rods being fixed to a block or refractory material by means of U-shaped fixing elements, the block being provided with a central opening into which a metallic fixing bolt can be inserted from the inside of the block, said bolt being joined to the metallic wall of the furnace by means of an electric-arc welding apparatus at one end, the other end being threaded and joined to the block of refractory material by screwing into an accessory disk. This known apparatus likewise requires that the furnace be entered for replacing a defective insulating block and, consequently, that the furnace be shut down for this purpose and allowed to cool.